Modern wireless communications standards require a mobile terminal to be capable of transmitting and receiving wireless signals over a wide range of frequencies. On its own, an antenna can generally only transmit and receive signals efficiently within a relatively narrow frequency band. Accordingly, tuning circuitry must be attached to the antenna in order to widen the frequency band over which an antenna can efficiently transmit and receive signals.
FIG. 1A shows an exemplary tunable patch antenna 10 for use in a mobile terminal. The tunable patch antenna 10 includes a substrate 12, an antenna surface 14, a grounding plane 16, an RF feed 18, and one or more tuning leads 20. The substrate 12 is located between the antenna surface 14 and the grounding plane 16. The RF feed 18 runs through the grounding plane 16 and the substrate 12 to contact the antenna surface 14. Each one of the tuning leads 20 runs between a point of the antenna surface 14 and a fixed impedance, such as the grounding plane 16.
In a transmit mode of operation, an RF signal is delivered to the RF feed 18. The RF signal runs past the grounding plane 16 and the substrate 12 to the antenna surface 14, where it is then radiated into the environment. By selectively placing one or more of the tuning leads 20 between a point of the antenna surface 14 and a fixed impedance, the impedance of the tunable patch antenna 10 can be changed. As the impedance of the tunable patch antenna 10 is changed, the transmission characteristics of the tunable patch antenna 10 similarly change. Accordingly, by selectively placing one or more of the tuning leads 20 in contact with a fixed impedance, a “sweet spot” having a maximum efficiency for transmission of a signal about a given frequency band may be found, thereby enabling the tunable patch antenna 10 to efficiently operate over a wider range of transmission frequencies than would otherwise be possible.
In a receive mode of operation, an RF signal arrives at the antenna surface 14 from the surrounding environment. The RF signal is passed from the antenna surface 14 to the RF feed 18, where it is subsequently delivered to one or more components in the front end circuitry of a mobile terminal (not shown). By selectively placing one or more of the tuning leads 20 between a point of the antenna surface 14 and a fixed impedance, the impedance of the tunable patch antenna 10 can be changed. As the impedance of the tunable patch antenna 10 is changed, the reception characteristics of the tunable patch antenna 10 similarly change. Accordingly, by selectively placing one or more of the tuning leads 20 in contact with a fixed impedance, a “sweet spot” having a maximum efficiency for reception of signals about a given frequency band may be found, thereby enabling the tunable patch antenna 10 to efficiently operate over a wider range of reception frequency than would otherwise be possible.
FIG. 1B shows a three-dimensional representation of the tunable patch antenna 10 shown in FIG. 1A. As shown in FIG. 1B, each one of the tuning leads 20 is spread out over the width of the antenna surface 14, and includes conventional antenna tuning switch circuitry 22 for selectively placing the tuning lead 20 in contact with a fixed impedance such as the grounding plane 16. As discussed above, as each one of the tuning leads 20 are placed in contact with a fixed impedance, the transmission and reception characteristics of the tunable patch antenna 10 are changed, thereby allowing for efficient transmission and reception of RF signals across a wide range of frequencies.
Although effective at altering the transmission and/or reception characteristics of the tunable patch antenna 10, the conventional antenna tuning switch circuitry 22 coupled to each one of the tuning leads 20 may itself degrade the performance of the tunable patch antenna 10. For example, the conventional antenna tuning switch circuitry 22 attached to each one of the tuning leads 20 may introduce insertion loss in the signal path of the antenna in the ON state, may introduce parasitic capacitance and/or inductance in the signal path of the antenna in the OFF state, and may be prone to breaking down, thereby causing one or more of the tuning leads 20 to unintentionally turn ON and change the transmission and/or reception characteristics of the tunable patch antenna 10.
FIG. 2 shows conventional antenna tuning switch circuitry 22 for use in the tunable patch antenna 10 shown in FIGS. 1A and 1B. The conventional antenna tuning switch circuitry 22 includes a semiconductor die 24, a shunt switch SH_SW, an electrostatic discharge (ESD) protection switch ESD_SW, an input port 26, a first ground port 28, a second ground port 30, control circuitry 32, a control port 34, a supply voltage port 36, and an off-die ground 38. The shunt switch SH_SW is coupled between the input port 26 and the first ground port 28. The ESD protection switch ESD_SW is coupled between the first ground port 28 and the second ground port 30. The control circuitry 32 includes a control signal input port 40 coupled to the control port 34, a ground connection port 42 coupled to the second ground port 30, a switch driver output port 44 coupled to the shunt switch SH_SW, and a supply voltage input port 46 coupled to the supply voltage port 36. The shunt switch SH_SW, the ESD protection switch ESD_SW, the input port 26, the first ground port 28, the second ground port 30, the control circuitry 32, and the control port 34 are integrated on the semiconductor die 24. The first ground port 28 is coupled to the off-die ground 38 via a first ground connection 48. The second ground port 30 is coupled to the off-die ground 38 via a second ground connection 50.
In operation, the input port 26 of the conventional antenna tuning switch circuitry 22 is coupled to a point on the antenna surface 14 of the tunable patch antenna 10 through one of the tuning leads 20. In order to alter the transmission and/or reception characteristics of the tunable patch antenna 10 in response to one or more control signals CTL_SIG received at the control port 34, the control circuitry 32 may close the shunt switch SH_SW to couple the input port 26 to a fixed impedance, such as ground. Due to the differences in the parasitic inductance of the first ground connection 48 and the second ground connection 50, a potential voltage difference may exist between the first ground port 28 and the second ground port 30 over a variety of operating conditions. Accordingly, the impedance of the path from the input port 26 through the control circuitry 32 to the second ground port 30 has the potential to be lower than the path from the input port 26 through the shunt switch SH_SW to the first ground port 28. Due to the potential for a low impedance path to be presented through the control circuitry 32, should an ESD event occur, it may pass through the control circuitry 32 to the second ground port 30, thereby damaging or disabling the control circuitry 32. Accordingly, the ESD protection switch ESD_SW is necessarily included in the conventional antenna tuning switch circuitry 22 to couple the first ground port 28 and the second ground port 30 in the event of an ESD event so that the lowest impedance path to ground is provided around, rather than through, the control circuitry 32.
FIG. 3A shows a simplified equivalent circuit for the conventional antenna tuning switch circuitry 22 when the conventional antenna tuning switch circuitry 22 is in an ON state of operation. As shown in FIG. 3A, the ON impedance of the conventional antenna tuning switch circuitry 22 can be represented by the following equation:ZON=ZON_SH+[(ZOFF_ESD+LC2)//LC1]  (1)where ZON_SH is equal to the ON impedance of the shunt switch SH_SW, ZOFF_ESD is equal to the parasitic impedance of the ESD protection switch ESD_SW when the ESD protection switch ESD_SW is in the OFF state, LC2 is equal to the equivalent inductance of the second ground connection 50, and LC1 is equal to the equivalent inductance of the first ground connection 48. As shown by FIG. 3A and equation 1, the ESD protection switch ESD_SW, the first ground connection 48, and the second ground connection 50 may significantly contribute to the ON impedance of the conventional antenna tuning switch circuitry 22. The additional impedance provided by the ESD protection switch ESD_SW, the first ground connection 48, and the second ground connection 50 increases the insertion loss of the RF signal path in the tunable patch antenna 10, thereby degrading the quality of signals transmitted and received by the tunable patch antenna 10.
FIG. 3B shows a simplified equivalent circuit for the conventional antenna tuning switch circuitry 22 when the conventional antenna tuning switch circuitry 22 is in an OFF state of operation. As shown in FIG. 3A, the parasitic OFF state impedance of the conventional antenna tuning switch circuitry 22 can be represented by the following equation:ZOFF=ZOFF_SH+[(ZOFF_ESD+LC2)//LC1]  (2)where ZOFF_SH is equal to the parasitic OFF state impedance of the shunt switch SH_SW, ZOFF_ESD is equal to the parasitic impedance of the ESD protection switch ESD_SW when the ESD protection switch ESD_SW is in the OFF state, LC2 is equal to the equivalent inductance of the second ground connection 50, and LC1 is equal to the equivalent inductance of the first ground connection 48. As shown by FIG. 3B and equation 2, the ESD protection switch ESD_SW, the first ground connection 48, and the second ground connection 50 may significantly contribute to the parasitic OFF state impedance of the conventional antenna tuning switch circuitry 22. The additional impedance provided by the ESD protection switch ESD_SW, the first ground connection 48, and the second ground connection 50 increases the insertion loss of the RF signal path in the tunable patch antenna 10, thereby degrading the quality of signals transmitted and received by the tunable patch antenna 10.
Accordingly, a tuning switch for a tunable patch antenna 10 is required that is capable of providing a low impedance path to a fixed impedance in the ON state, introducing a low parasitic load on the antenna in the OFF state, and handling high amplitude signals.